1. Field of the Invention
The present invention relates to an automatic gain control circuit for a receive apparatus for a mobile object.
2. Description of the Related Art
A car, for example, as a mobile object has an indoor-attachment-type, film-type antenna installed thereto. Along therewith, a variable gain amplifier (VGA) with an automatic gain control (AGC) circuit that amplifies the received signal of this antenna and outputs the signal to a general-purpose radio tuner and the like (receive apparatus) at the rear stage is installed in the car.
This variable gain amplifier with the automatic gain control circuit has a well-known AGC circuit connected to the output of the variable gain amplifier VGA as shown in FIG. 5. This AGC circuit performs feed-back control so as to hold the output level of the variable gain amplifier constant by following the variations in the electrical field intensity of the received signal of the antenna. See, for example, Japanese Patent Laid-open Publication No. 2003-125012.
In the above related art, however, in order to achieve the AGC function, the AGC circuit performs feed-back control, i.e., the gain adjustment of the variable gain amplifier VGA persistently based on the output of the variable gain amplifier VGA.
Hence, when the received signal is large (at a large input), the gain of the variable gain amplifier VGA becomes large and the output signal thereof becomes distorted, thereby possibly causing the occurrence of interference on the receive apparatus side.
Furthermore, with the feed-back control scheme wherein the gain is adjusted according to the output of the variable gain amplifier VGA, the problem occurs that the control range is limited to the output range-of the variable gain amplifier. Moreover, for example, in the case where the antenna is attached to a car, the intensity of the received signal possibly changes rapidly, and hence in the feed-back control scheme, the response may be delayed. Thus, variations in receive sensitivity may be caused.